To explain a bit more in-depth... AMD GCN hardware only allows you to pass 16 DWORD values (64 bytes) as the arguments to a shader. The root signature describes a structure that can fit into this 64byte area.
It's maybe worth to add, that the root signature has a limit of 64 DWORDs. So it can easily overflow the 64 byte limit GCN hardware sets for user data registers. Therefore it's a good idea to keep the root signature size small (at least on this hardware). Otherwise parts are spilled in 'slower' memory and a additional indirection is needed for resolve.